On the generation and propagation of the southern African coastal low

The time‐height variability of the lower marine atmosphere during the eastwards propagation of coastal lows around the coast of southern Africa is presented. These coastal lows are shallow, mesoscale disturbances that are trapped vertically by a strong subsidence inversion and horizontally, through Coriolis effects, against the steep escarpment that fringes the subcontinent. Prior to the event, warm offshore flow descending off this escarpment at about the inversion level provides an input of buoyancy which, together with the cyclonic vorticity acquired during the descent, acts to generate a coastal low rather than a high. Analysis of the data shows that coastal lows are characterized by a marked lowering of the inversion level, a switch in the direction of the winds below the inversion and a drop in the near‐surface temperature, which are all consistent with the hypothesis that the coastal low is an internal Kelvin wave propagating in the marine layer. Good agreement between the observed propagation speed of the coastal low and the theoretical phase speed of such a wave reinforces this hypothesis. A model of the vertical buoyancy advection into the marine atmosphere that results from the warm offshore flow is presented. It is illustrated that an internal Kelvin wave, in the form of a coastal low, results from this buoyancy advection. Comparisons are made between coastal‐low dynamics and those characteristics of other mesoscale, coastally‐trapped disturbances which propagate in the marine layers of California and south‐eastern Australia.